One of the goals of plant genetic engineering is to produce plants with agronomically important characteristics or traits. Recent advances in genetic engineering have provided the requisite tools to produce transgenic plants that contain and express foreign genes (Kahl et al., World J. of Microbiol. Biotech. 11:449–460, 1995). Particularly desirable traits or qualities of interest for plant genetic engineering would include but are not limited to resistance to insects, fungal diseases, and other pests and disease-causing agents, tolerances to herbicides, enhanced stability or shelf-life, yield, environmental tolerances, and nutritional enhancements. The technological advances in plant transformation and regeneration have enabled researchers to take exogenous DNA, such as a gene or genes from a heterologous or a native source, and incorporate the exogenous DNA into the plant's genome. In one approach, expression of a novel gene that is not normally expressed in a particular plant or plant tissue may confer a desired phenotypic effect. In another approach, transcription of a gene or part of a gene in an antisense orientation may produce a desirable effect by preventing or inhibiting expression of an endogenous gene.
In order to produce a transgenic plant, a construct that includes a heterologous gene sequence that confers a desired phenotype when expressed in the plant is introduced into a plant cell. The construct also includes a plant promoter that is operably linked to the heterologous gene sequence, often a promoter not normally associated with the heterologous gene. The construct is then introduced into a plant cell to produce a transformed plant cell, and the transformed plant cell is regenerated into a transgenic plant. The promoter controls expression of the introduced DNA sequence to which the promoter is operably linked and thus affects the desired characteristic conferred by the DNA sequence.
It would be advantageous to have a variety of promoters to tailor gene expression such that a gene or gene(s) is transcribed efficiently at the right time during plant growth and development, in the optimal location in the plant, and in the amount necessary to produce the desired effect. For example, constitutive expression of a gene product may be beneficial in one location of the plant but less beneficial in another part of the plant. In other cases, it may be beneficial to have a gene product produced at a certain developmental stage of the plant or in response to certain environmental or chemical stimuli. The commercial development of genetically improved germplasm has also advanced to the stage of introducing multiple traits into crop plants, often referred to as a gene stacking approach. In this approach, multiple genes conferring different characteristics of interest can be introduced into a plant. It is important when introducing multiple genes into a plant that each gene is modulated or controlled for optimal expression and that the regulatory elements are diverse in order to reduce the potential of gene silencing. In light of these and other considerations, it is apparent that optimal control of gene expression and regulatory element diversity are important in plant biotechnology.